The present invention relates to a ruthenic acid compound having a novel structure, a production method thereof and an electrochemical device using the ruthenic acid compound.
Electrical double-layer capacitors, pseudo electrical double-layer capacitors, secondary batteries and memory devices are typical examples of an electrochemical device having power storage capability. Depending on their characteristics, these devices have been applied to practical equipment. Due to its higher power density and longer life than the secondary battery, the electrical double-layer capacitor has been used in backup power sources requiring high reliability. The secondary battery, which is the most typical and classic power storage device, has higher energy density than the electrical double-layer capacitor. However, the secondary battery is shorter in life than the electrical double-layer capacitor and has to be replaced with a new one after a certain use period. On the other hand, the memory device has already been commercialized as a reusable timer, but development of a long-term timer running for 10 to 15 years has been waited for.
These power storage devices are different in characteristic due to their mechanisms for storing electric power. The pseudo electrical double-layer capacitor generates a capacitance by electrochemical adsorption at an interface between the surface of metal oxide such as RuO2, IrO2 or Co3O4 and an electrolyte. This capacitance is called pseudo-capacitance and distinguished from an electrical double-layer capacitance generated at an interface between the surface of an activated carbon electrode and an electrolyte.
The electrical double-layer capacitor does not cause an electrochemical reaction at the interface between the electrode and the electrolyte, but only brings about movement of ions contained in the electrolyte. Accordingly, the capacitor is not as easily deteriorated as the secondary battery, permitting long life and high power density due to the movement of ions at high speed.
On the other hand, the secondary battery is apt to deteriorate easily through charge/discharge because it utilizes the electrochemical reaction between the electrode and the electrolyte. The secondary battery causes the chemical reaction at a relatively low speed and has short battery life and relatively low power density. However, since the electrode material itself stores the energy in the form of chemical energy, the secondary battery shows higher energy density than the electrical double-layer capacitor that stores the energy only at the interface between the electrode and the electrolyte.
In view of these conventional power storage devices, there has been proposed a pseudo electrical double-layer capacitor having high power density and long life of the electrical double-layer capacitor and high energy density of the secondary battery. In general, ruthenium, iridium and cobalt are typical materials constituting an electrode of the pseudo electrical double-layer capacitor. However, since these materials are expensive, reduction in cost and increase in performance have been challenges to be solved.
As solutions to the challenges, there have been proposed an electrode dispersing rutile-type RuO2 therein, an electrode material comprising a ruthenium compound and a vanadium compound adsorbed on activated carbon (Japanese Laid-Open Patent Publication No. HEI11-354389) and an electrode comprising a solid solution containing a ruthenium compound and a vanadium compound.
Further, a layered ruthenic acid compound has been proposed as a material for a pseudo electrical double-layer supercapacitor (Y. Takasu et al., Electrochim, Acta 2000, 45, 4135).
If a ruthenium compound, which is expensive, is supported on the surface of a host material such as activated carbon, the ruthenium compound will have a rutile-type or amorphous crystal structure. Accordingly, the initial voltage is expected to be high, but the ruthenium compound may come off the host material through repetitive charge/discharge for a long time.
Further, if a vanadium compound is added, bond strength to the host material decreases, causing dropping of the ruthenium compound through the long-term charge/discharge. Thus, a problem of battery life remains to be solved.
So far, a layered compound such as graphite has received attention as an electrode material. Typically used layered compounds are graphitizable carbon, TiS2, MoS2, CoO2 and V6O13, whereas the synthesis of a layered ruthenium compound has been difficult.
The above-mentioned related document Y. Takasu et al., Electrochim, Acta 2000, 45, 4135 discloses a layered ruthenic acid compound. According to this document, the layered ruthenic acid compound is applied to a titanium metal current collector to form an electrode. However, the obtained electrode is not satisfactory due to lack of strength.
In view of the above, the present invention intends to provide a novel layered ruthenic acid compound having an increased active area and significantly improved charging capability.
In another aspect, the present invention intends to provide an electrode capable of realizing a high-powered large-capacitance electrochemical capacitor by using the layered ruthenic acid compound and an electrochemical device using the electrode.